A cylindrical body such as a photosensitive drum substrate is required to have high surface accuracy, and therefore surface inspection is conducted to screen out cylindrical bodies having surface defects, such as, e.g., scratches, irregularities, adherence of foreign substances, and/or stains.
For example, in a surface inspection device for a cylindrical body disclosed in the below listed Patent Document 1, as shown in FIG. 8, it is configured such that illumination light L is irradiated on a cylindrical body W from a light source 101 disposed laterally upward of the cylindrical body W, while the regular reflection light Lr (reflection light image) is captured with a camera 102, so that surface defects of the cylindrical body W are detected based, on the captured image data.
In recent years, as a photosensitive drum substrate constituting a cylindrical body W to be inspected, a substrate obtained by drawing an extruded tube has been widely used. As shown in FIG. 9, in a cylindrical body W obtained by drawing, portions that could become defective such, as irregularities on a surface, in most cases, are formed into streak-like shapes extending parallel to the axial direction (longitudinal direction) X. Among these streak-like concave portions D and/or protruding portions, a comparatively shallow streak-like concave portion Da will not become defective, while a steak-like concave portion Db relatively deep in depth or having a standing edge at the circumference thereof will become defective. Thus, in cases where there exist such defective streak-like concave portions Db, the substrate is treated as a defective one,